The Germans proved to be rather inefficient engineers. Yes, they could produce some of the best weapons of the war, but insisted on adding so many minor refinements and using so many different models that any sort of mass production was impossible. The United States, on the other hand, simplified its production line. IIRC, the US Army ended up using only three different types of engines. Everything was made as simple as possible. So, at the end, the Germans were left with a WWI-style army with a few points of modernity (the panzer divisions) while the entire US army (~90 divisions) was mechanized (with shiploads of material being exported to allied nations).